US4661775A - Chemical shift imaging with field inhomogeneity correction - Google Patents

Chemical shift imaging with field inhomogeneity correction Download PDF

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US4661775A
US4661775A US06/755,120 US75512085A US4661775A US 4661775 A US4661775 A US 4661775A US 75512085 A US75512085 A US 75512085A US 4661775 A US4661775 A US 4661775A
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chemical shift
data set
image
components
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Donald W. Kormos
Hong-Ning Yeung
Henry S. Dewhurst
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Siemens AG
Ethicon Inc
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Technicare Corp
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Priority to US06/755,120 priority Critical patent/US4661775A/en
Priority to JP61165445A priority patent/JPH074349B2/ja
Priority to AT86305423T priority patent/ATE65847T1/de
Priority to EP86305423A priority patent/EP0210038B1/fr
Priority to DE8686305423T priority patent/DE3680600D1/de
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    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01R—MEASURING ELECTRIC VARIABLES; MEASURING MAGNETIC VARIABLES
    • G01R33/00—Arrangements or instruments for measuring magnetic variables
    • G01R33/20—Arrangements or instruments for measuring magnetic variables involving magnetic resonance
    • G01R33/44—Arrangements or instruments for measuring magnetic variables involving magnetic resonance using nuclear magnetic resonance [NMR]
    • G01R33/48—NMR imaging systems
    • G01R33/54—Signal processing systems, e.g. using pulse sequences ; Generation or control of pulse sequences; Operator console
    • G01R33/56—Image enhancement or correction, e.g. subtraction or averaging techniques, e.g. improvement of signal-to-noise ratio and resolution
    • G01R33/565—Correction of image distortions, e.g. due to magnetic field inhomogeneities
    • G01R33/56563—Correction of image distortions, e.g. due to magnetic field inhomogeneities caused by a distortion of the main magnetic field B0, e.g. temporal variation of the magnitude or spatial inhomogeneity of B0
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01R—MEASURING ELECTRIC VARIABLES; MEASURING MAGNETIC VARIABLES
    • G01R33/00—Arrangements or instruments for measuring magnetic variables
    • G01R33/20—Arrangements or instruments for measuring magnetic variables involving magnetic resonance
    • G01R33/44—Arrangements or instruments for measuring magnetic variables involving magnetic resonance using nuclear magnetic resonance [NMR]
    • G01R33/48—NMR imaging systems
    • G01R33/483—NMR imaging systems with selection of signals or spectra from particular regions of the volume, e.g. in vivo spectroscopy
    • G01R33/485—NMR imaging systems with selection of signals or spectra from particular regions of the volume, e.g. in vivo spectroscopy based on chemical shift information [CSI] or spectroscopic imaging, e.g. to acquire the spatial distributions of metabolites

Definitions

  • This invention relates to nuclear magnetic resonance (NMR) imaging and, in particular, to the formation of NMR images of two chemical shift components.
  • a first image would be formed of a plane of the body showing only the varying content of fat in the plane.
  • a second image would be formed showing only the content of water in the plane. Comparison of the two images would yield clinically useful information about the region being examined.
  • spin echo signals are generated by first tipping the spin isochromats, or spin vectors, with a 90° radio frequency (RF) pulse into a plane transverse to the static magnetic field. After a period of delay ⁇ , the spin components having different frequencies have begun to dephase, or spread out in the transverse plane. The spins are then flipped in the transverse plane with a 180° RF pulse. This reverses the trajectories of all the spin isochromats causing them to rephase to form a spin echo signal (SE).
  • SE spin echo signal
  • spin dephasing and rephasing can also be affected by the application of magnetic field gradients, the presence of which is required to enable spatial information to be encoded in the NMR signal.
  • spin echo signals can be produced by applying magnetic field gradients in a controlled manner. For instance, after the spin vectors are brought to the transverse plane by a 90° RF pulse, if one turns on a field gradient for a short duration, the spin isochromats at different locations along the direction of the gradient will be dephased by the field gradient pulse. In rephasing the isochromats to form an echo, one could either reverse the gradient sense or turn the gradient back on in the same direction after a 180° RF pulse.
  • a gradient spin echo signal (GSE) is formed.
  • GSE gradient spin echo signal
  • An alternate means of obtaining NMR data is to collect FID signal data using gradient reversals.
  • the FID (free induction decay) signal from the time T E and on is recorded during a steady "read" or frequency encoding gradient.
  • the read gradient cannot be turned on instantly, it is normally switched on at an earlier time than T E . Its direction is then reversed and brought to the steady value before T E .
  • the timing and the magnitude of the reversal gradient the spin isochromats along the direction of the gradient can be refocused at T E .
  • the images obtained from SE signals have a distinct advantage over those obtained from FID signals.
  • the chemical shift and field inhomogeneity information will be completely masked by the applied field gradient. Their only effects will be manifested as chemical shift and field inhomogeneity artifacts. These artifacts are more severe for images formed from FID signals after gradient reversal as compared to those formed from SE signals.
  • both types of images suffer from geometrical as well as intensity distortion caused by field nonuniformity and pixel mis-registration caused by chemical shifts
  • the images of the FID signals suffer an additional phase distortion which stems from the dephasing effects of field inhomogeneities during the time required to spatially encode and refocus the spin isochromats.
  • the asymmetrical nature of FID signals mandates the use of phase-sensitive image reconstruction, whereas the symmetrical nature of the SE signals permits the use of magnitude reconstruction.
  • phase factor term e i ⁇ where ⁇ is proportional to the product of 2 ⁇ s and the field inhomogeneity. If ⁇ s is adjusted such that the two components are 180° out-of-phase, the contributions of the two components to the image intensity can be, to some extent, separated by the following means.
  • the Dixon technique would enable one to form two separate images, of water and fat, for instance.
  • field inhomogeneities are always present in NMR systems due to various sources.
  • magnitude images present a problem, much like the one encountered in T 1 determination using magnitude data from an inversion-recovery experiment: the sign of each data point is lost.
  • the two components are not then segregated into two separate images. Instead, one image depicts the more plentiful component at each pixel, while the other depicts the less plentiful component.
  • the more plentiful component image will be referred to hereafter as the major component image and the less plentiful the minor image.
  • Dixon's technique is an excellent advance in chemical shift imaging, it falls short of being able to present truly separate images of the two components in a typical magnetic field with field inhomogeneities.
  • a technique for forming truly separate NMR images of two chemical shift components.
  • a conventional spin echo experiment is performed in which spin echo signals are generated with the SE and the GSE being coincident in time.
  • Two further spin echo experiments are performed, each with their 180° RF pulses time shifted with respect to 1/2T E such that, at time T E , the two chemical shift components are phase shifted with respect to each other by different multiples of ninety degrees.
  • the three images are each represented by spatially variable complex numbers of the general form:
  • f A and f B are the relative weightings of the chemical shift components
  • ⁇ is the relative phase angle of the two components
  • ⁇ is a spatially dependent phase angle which can be a function of instrumentation errors and local field inhomogeneity.
  • the three images exhibit three different types of information content.
  • the conventional spin echo image contains virtually no chemical shift and field inhomogeneity information, but may contain an instrumentation error phase angle.
  • the second image contains chemical shift and field inhomogeneity information, and may also contain an instrumentation error phase angle.
  • the third image contains field inhomogeneity information and an instrumentation error phase angle comparable to that of the first image. By combining the information of the first and third images the field inhomogeneity information can be determined. This field inhomogeneity information is then used to correct the second image, removing field non-uniformity factors from its image content. The corrected second image is combined with the first image to separately identify the chemical shift components.
  • phase correction can be performed using a phantom object, if desired, or by use of the imaging object in situ.
  • the principles of the present invention are also applicable to in situ or phantom magnetic field mapping.
  • FIG. 1 illustrates two chemical shift components which are to be separately imaged
  • FIG. 2 illustrates spin isochromats in various stages during chemical shift imaging
  • FIG. 3 illustrates pulse sequences and spin vectors used to perform chemical shift imaging in accordance with the Dixon technique
  • FIG. 4 illustrates pulse sequences used to perform chemical shift imaging in accordance with the principles of the present invention
  • FIG. 5 illustrates chemical shift component vectors during various stages of the chemical shift imaging experiment of FIG. 4;
  • FIG. 6 is a flowchart of the processing used to form images of the two chemical shift components of FIG. 1 in accordance with the principles of the present invention
  • FIG. 7 illustrates field inhomogeneity effects across an image plane
  • FIG. 8 illustrates a phase range table associated with FIG. 7
  • FIG. 9 vectorially represents chemical shift component vectors in phase quadrature orientation for chemical shift imaging.
  • imaging components A and B may be water and fat.
  • the presence of water may be indicated by resonance of the hydrogen protons in the H 2 O molecule, and the presence of fat may be indicated by resonance of the hydrogen protons of the methylene component (--CH 2 --) of fat.
  • a typical resonance line for water is shown in FIG. 1 of frequency ⁇ A , which is approximately 4.8 part per million (ppm).
  • a resonance line for fat is given at frequency ⁇ B as approximately 1.3 ppm. Both resonance lines are relative to a tetramethyl silane (TMS) reference line indicated as REF.
  • TMS tetramethyl silane
  • NMR imaging it is the relation of the two chemical shift components to each other, in this case a separation of 3.5 ppm, which is significant. This difference in chemical shift can be generally expressed as ⁇ .
  • a reference frequency for the NMR imaging apparatus ⁇ o , is also shown in the FIGURE. Its position is arbitrarily shown between the two components and closer to the A component.
  • FIG. 2 illustrates the various positions of the A and B component spin vectors during an experiment in accordance with the Dixon technique of chemical shift imaging.
  • x and y coordinate axes are shown for a transverse (x-y) plane 10 of a rotating reference frame.
  • the z axis extends perpendicular to the drawings.
  • the reference frequency ⁇ o is between the two components they will precess in opposite directions as shown by the small rotational arrows on each vector. And since the B component is more widely separated from the ⁇ o reference frequency than the A component, the B component will precess faster than the A component.
  • pulse sequences are shown which may be used to perform a Dixon type chemical shift experiment.
  • a 90° RF pulse 20 is applied to a subject in the static magnetic field.
  • a first G x gradient pulse 26 is applied, which has an integral value equal to that of the subsequent "read" gradient 28 up to the point in time T E .
  • a 180° RF pulse 22 is applied to generate a spin echo signal.
  • the spin echo signal 24 is centered around a point in time which is ⁇ 1 later than the 180° RF pulse 22.
  • the spin echo signal is sampled in the presence of the G x read gradient 28.
  • Each acquired signal represents the vector sum shown at the bottom of FIG. 3a, where the magnitudes of the A and B vectors are aligned with each other and are offset from the real axis r of the complex plane i-r by an angle ⁇ 1 , an instrumental phase factor which is generally spatially nonuniform.
  • FIG. 3b A second sequence of data acquisition is illustrated in FIG. 3b.
  • a 90° RF pulse 30 is applied to the subject, followed by a G x gradient pulse 36.
  • a 180° RF pulse 32 is applied.
  • a time ⁇ s after the 180° RF pulse 32 marks the middle of the interval between the 90° pulse 30 and the subsequent spin echo signal, 1/2T E .
  • T E which is equal to 2 ⁇ 2 +2 ⁇ s
  • the spin echo signal 34 develops, which is sampled in the presence of the G x read gradient 38.
  • the 2 ⁇ s time is chosen such that ⁇ is equal to 180° ( ⁇ radians) in the above expression. As shown at the bottom of FIG.
  • the A and B vectors at time T E are at an angle of 180° to each other and are rotated from the real axis by an angle ⁇ 2 .
  • This phase angle is the sum of ⁇ 1 and another phase angle representing static field inhomogeneity.
  • the sequence of FIG. 3b is repeated a sufficient number of times with variations for the acquisition of data for subsequent Fourier transform processing and image formation.
  • E(x,y) is seen to be a function of spatial location, x and y. This is because the field inhomogeneity can vary in the static magnetic field from one spatial location to another. Thus, E(x,y) cannot be determined in the above expression because, in the case where f A equals f B , the second expression is equal to zero: there is no signal to measure.
  • Dixon's technique multiplies the above expression by its complex conjugate to eliminate the exponential terms, which gives a value
  • the major (sum) image and the minor (difference) image are then formed from the magnitudes:
  • lines (a) through (e) represent a first sequence and lines (d) through (h) represent a second sequence.
  • a 90° RF pulse 40 is applied to the subject in the presence of a G z gradient 70 for selecting the planar slice which is to be imaged.
  • an amplitude variable G y gradient 80 is applied and a G x gradient 60 is applied.
  • the first sequence is repeated numerous times to acquire sufficient data for subsequent Fourier transformation, each time varying the amplitudes of the G y gradients 80 and 82.
  • the pulses and gradients of the second sequence are similar to those of the first, up until the time of application of the first 180° pulse 42.
  • This pulse is applied at time ⁇ 2 , which is earlier than T E /2 by a time ⁇ s .
  • the desired first spin echo signal 44' is centered in time around time T E , which is 2 ⁇ s in time later than time 2 ⁇ 2 .
  • the spin echo signal 44' is sampled during a sampling period 50' in the presence of a G x read gradient 62'.
  • the following G y gradient 82 and 180° pulse 46 are applied as in the first sequence, except for the earlier occurrence of the 180° pulse 46 at time 3 ⁇ 2 . Again the time of acquisition of the next spin echo signal is changed.
  • the spin echo signal 48' is centered in time at time 2T E , which is 4 ⁇ s later than time 4 ⁇ .sub. 2.
  • the spin echo signal 48' is sampled during period 52' in the presence of G X read gradient 64'.
  • the second sequence is repeated with variation of Gy gradient amplitudes 80 and 82 for subsequent Fourier transform processing.
  • the ⁇ s delay can be either positive or negative with respect to multiples of T E /2.
  • Pulse 42 could also occur at time (T E /2)+ ⁇ s , which would cause time 2 ⁇ 2 to occur at time T E +2 ⁇ s .
  • the spin echo signal 44' would continue to be centered at time T E , which is equal to 2 ⁇ 2 -2 ⁇ s .
  • spin echo signal 48' would then be centered at time 4 ⁇ 2 -4 ⁇ s .
  • the characteristics of the complex images formed by spin echo signals 44, 48, 44', 48' are vectorally represented in FIGS. 5a-5d.
  • the magnitudes of each component A and B are shown as vectors in a complex plane.
  • the angles ⁇ INST and ⁇ ' INST shown are instrumental phase delays caused by RF absorption of the subject as well as eddy current effects of gradient switching.
  • ⁇ E is an additional phase angle caused by static field inhomogeneity.
  • the component magnitudes A and B, as well as the phase angles ⁇ INST and ⁇ ' INST , and ⁇ E are spatially dependent.
  • FIG. 5a representing spin echo signal 44
  • the A and B spin components are in alignment (in-phase) and make an angle ⁇ INST relative to the real axis of the complex plane.
  • FIG. 5b represents spin echo 48.
  • the A and B spin vectors are again aligned but rotated from the real axis by a different phase angle, ⁇ ' INST .
  • FIG. 5c depicts spin echo signal 44'. With ⁇ s chosen for a 180° angle between the spin components A and B, the vectors are shown as antiparallel, or 180° out-of-phase.
  • the phase angle relative to the real axis is now the sum of ⁇ INST (as in FIG.
  • One objective of the present invention is to remove the field inhomogeneity phase angle, ⁇ E , from the complex image represented by FIG. 5c. This will allow separation of the desired A and B component images by the complex sum and difference of images represented by FIG. 5a and a corrected FIG. 5c image.
  • a complex image with the A and B components in-phase and rotated from the real axis by ⁇ E can mathematically be obtained from the second echo images depicted in FIGS. 5b and 5d.
  • This intermediate image can then be used to remove the ⁇ E rotation in the complex image diagrammed in FIG. 5c.
  • This intermediate image in fact represents an in situ image of the static field inhomogeneity on a pixel-by-pixel basis.
  • FIGS. 5a and 5c alone cannot be used to identify and remove the ⁇ E rotation. This is because the A and B components of the two images are in-phase and out-of-phase.
  • the ⁇ E rotation can be identified when the other components of the complex images, including the A and B vector alignment and ⁇ INST phase errors, are in correspondence and will cancel through complex division. That is the case in FIGS. 5b and 5d, which differ only in the field inhomogeneity component 2 ⁇ E .
  • the spin echo signal information is processed as indicated in the flowchart of FIG. 6.
  • Spin echo signal information from the first sequence of FIG. 4 is stored as data set 1 in a computer memory.
  • the spin echo information from the second sequence is stored as data set 2.
  • the echo 2 information in data set 1 is of the form
  • f A and f B are the weighting factors of the A and B components, dependent upon the relaxation times as well as the nuclear density of components A and B.
  • the echo 2 information in data set 2 is of the form
  • the field range of ⁇ (x,y) terms is thereby a spatial phase representation of the magnetic field.
  • correction may then be performed on the echo 1 information of data set 2. This is accomplished by complex multiplication of the image representing echo 1 in data set 2 and the intermediate image representing ⁇ E .
  • the result of this correction is that the echo 1 data in data set 2 represents an image with a magnitude of A-B.
  • the echo 1 data in data set 1 represents an image with a magnitude of A+b.
  • the vectors representing these two images are colinear, having a common phase angle of ⁇ INST with respect to the real axis r.
  • a further factor should be considered when performing the field image phase correction described above, which is the total phase shift due to field inhomogeneities throughout the image plane.
  • ⁇ E field is determined at any particular point, it is represented by a complex number with a phase angle in radians. This phase angle can only be represented in values within a 2 ⁇ radian range; in practice, the range is ideally bounded between - ⁇ and + ⁇ radians.
  • the image it is possible for the image to have true phase values exceeding several of these ranges, depending upon the chemical shift components being determined and the degree of the magnetic field nonuniformity and the subject being imaged.
  • the exemplary chemical shift components have a ⁇ separation of 3.5 ppm.
  • the field at one edge of an image plane was measured and found to be 6112.512 Gauss, and the field at the opposite edge of the image plane was found to be 6112.521 Gauss. The difference of these two figures is 0.009 Gauss.
  • image plane 10 of FIG. 7 a steady continuum of increasing field inhomogeneity extends from the lower left hand edge to the upper right hand edge as represented by arrows 100.
  • One complete range of phase values from - ⁇ to +90 radians is shown in the middle of the image. This range is separated from adjacent ranges at range boundaries 110 and 112.
  • ⁇ E The complex numerical terms of ⁇ E may be found as described above for any point along the continuum, and the phase angle for any term will be within a range of + ⁇ to - ⁇ radians.
  • the ⁇ E terms are of the form
  • E(x,y) contains a phase angle term in the range of - ⁇ to + ⁇ radians.
  • the phase angle terms will not distinguish which range they are in.
  • point 120 in FIG. 7 is located in a range just below boundary 110 where the range maximum of + ⁇ is located.
  • the phase of point 120 would be located as there indicated. Proceeding up the continuum and across boundary 110 in FIG. 7 a point 122 is found. In absolute terms, this point would have a phase value in excess of the + ⁇ boundary value; perhaps a value such as + ⁇ +( ⁇ /5) radians. In the range table of FIG. 8, this phase value is as shown by point 122.
  • This wrap around problem creates ambiguity in the complex numerical terms of the data set comprising the field image ⁇ E .
  • One way in which the ambiguity manifests itself is when the 2 ⁇ E data set from data set 2, echo 2 is to be divided by two to produce the ⁇ E intermediate field image for correction. Since the numbers in the data set are complex and there is a factor of 2 in the exponential, the mathematical function performed is in actuality a square root taken of complex numbers. The result of this square root function will be indeterminate when the set of data values encompasses more than one 2 ⁇ radian range.
  • Every image may be expected to have regions where the rate of change of the field inhomogeneity increases or decreases, in effect producing "hills and valleys" in a map of the phase field.
  • One such typical region is indicated at 102 in FIG. 7, where range boundaries 114 and 116 appear in quick, circular succession. Rates of field change can also increase through one region, then decrease through the next. For instance, in a transverse image through the human abdomen containing a portion of the liver, a sudden increase in the rate of field change was found in the vicinity of the liver. It is speculated that this phenomenon may be due to the high concentration of ferrous ions in the liver and the resultant effect of these ions on the local magnetic field. In any event, every living subject may be expected to have its own unique phase characteristics for any given image plane.
  • Unwrapping is achieved by picking a starting reference point in the field map data set, then plotting, point by point, the change in the field from one point to an adjacent point.
  • the point by point comparison should be done both horizontally and vertically through the field map so that the "hills and valleys" are all properly detected.
  • phase change values accumulate to a + ⁇ or - ⁇ value at a boundary, a boundary crossing will be detected.
  • a numerical correction of multiples of ⁇ 2 ⁇ radians for subsequent points across the boundary would then be applied to these points, thereby indicating that the subsequent points are in a different 2 ⁇ range.
  • the numerical correction factors are added or subtracted as appropriate to their respective phase value data points. If the process was performed properly, the result of this correction is a smoothly varying field map. By taking the respective correction factors into consideration when the complex numerical computations are performed, the results of each computation will be a determinate number. Without unwrapping, for instance, portions of the A component image will appear in the B component image, and vice versa, when the vectorial sums and differences are taken for image formation.
  • chemical shift images may be produced from two multi-spin echo sequences, the latter rotating the desired component vectors by 180° and 360°.
  • a first spin echo experiment can be performed in the conventional manner, with the 180° RF pulse coincident in time with 1/2T E .
  • a second experiment is performed with the 180° RF pulse time shifted with respect to 1/2T E so that at time T E the chemical shift components are rotated with respect to each other by some multiple of ⁇ radians, e.g., 180°.
  • the 180° RF pulse time is shifted with respect to 1/2T E such that at time T E the chemical shift components are rotated with respect to each other by some multiple of 2 ⁇ radians, e.g., 360°.
  • the signal information acquired in the three experiments will exhibit comparable ⁇ INST terms, thereby permitting identification of the field inhomogeneity information ⁇ E by combining the image data sets of the first and third experiments.
  • the ⁇ E information is then used to remove the field inhomogeneity factor from the second image data set, and the first image data set and the corrected second image data set may then be combined to produce separate images of the two chemical shift components.
  • This particular embodiment will find application principally in NMR imaging systems with low field strengths. In such systems, the technique of the preferred embodiment of FIGS.
  • the three single echo technique thus would provide a higher signal-to-noise field correction image, but at the expense of greater image acquisition time.
  • the multiple spin echo technique of the preferred embodiment is more desirable in most applications because only two experiments are required, which makes the total imaging time equivalent to that of the Dixon technique.
  • the multiple spin echo technique is preferable because the first spin echo signals of each sequence will yield chemical shift separation information and the second spin echo signals will yield field correction information. The necessary information for correction and presentation is thus acquired quickly and efficiently.
  • ⁇ E field image information in situ that is, from the subject itself
  • the ⁇ E field image data obtained by imaging the phantom would then be used to correct the spin echo information acquired from the subject.
  • each sequence could be a single echo sequence.
  • unique field effects of the subject such as the phase phenomena of the liver, would not be duplicated in the phantom and could result in less optimally corrected images.
  • the techiques of the present invention are applicable in situations where it is desirable to map the field inhomogeneity in the magnetic field of an NMR system.
  • the ⁇ E field image information itself is useful when information as to the presence of field inhomogeneities, either in a phantom or in situ, is sought, since its two dimensional presentation is a map of the magnetic field of the instrument.

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US06/755,120 1985-07-15 1985-07-15 Chemical shift imaging with field inhomogeneity correction Expired - Lifetime US4661775A (en)

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Application Number Priority Date Filing Date Title
US06/755,120 US4661775A (en) 1985-07-15 1985-07-15 Chemical shift imaging with field inhomogeneity correction
JP61165445A JPH074349B2 (ja) 1985-07-15 1986-07-14 フイ−ルド不均一情報を修正した化学シフト・イメ−ジ形成
DE8686305423T DE3680600D1 (de) 1985-07-15 1986-07-15 Bildgebung der chemischen verschiebung mit korrekturen der feldhomogenitaet.
EP86305423A EP0210038B1 (fr) 1985-07-15 1986-07-15 Imagerie du déplacement chimique avec des corrections de l'inhomogénéité de champ
AT86305423T ATE65847T1 (de) 1985-07-15 1986-07-15 Bildgebung der chemischen verschiebung mit korrekturen der feldhomogenitaet.

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Cited By (34)

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US4728893A (en) * 1985-07-31 1988-03-01 The Regents Of The University Of California Increased signal-to-noise ratio in magnetic resonance images using synthesized conjugate symmetric data
US4740753A (en) * 1986-01-03 1988-04-26 General Electric Company Magnet shimming using information derived from chemical shift imaging
US4761614A (en) * 1987-04-27 1988-08-02 Phospho-Energetics, Inc. Device and method for automatic shimming of NMR instrument
US4780673A (en) * 1987-10-05 1988-10-25 Varian Associates, Inc. Acquisition and processing of spin-echo NMR spectra
US4791369A (en) * 1986-07-14 1988-12-13 Hitachi, Ltd. Method of measuring magnetic field error of an NMR imaging apparatus and of correcting distortion caused by the error
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US4780673A (en) * 1987-10-05 1988-10-25 Varian Associates, Inc. Acquisition and processing of spin-echo NMR spectra
US4942359A (en) * 1988-01-29 1990-07-17 Hitachi, Ltd. Method of magnetic resonance imaging
DE3917007A1 (de) * 1988-05-24 1989-11-30 Toshiba Kawasaki Kk Magnetresonanz-abbildungsgeraet und magnetresonanz-abbildungsverfahren
US4868501A (en) * 1988-06-10 1989-09-19 Leland Stanford University Method and means for magnetic resonance spin-echo imaging using an adiabatic three pi pulse sequence
US5003263A (en) * 1988-12-14 1991-03-26 Hitachi, Ltd. , Hitachi Medical Corporation Image reconstruction method for a magnetic resonance imaging device and a device for realizing same
US5371465A (en) * 1991-03-13 1994-12-06 Hitachi, Ltd. Inspection method and apparatus using nuclear magnetic resonance (NMR)
US5617028A (en) * 1995-03-09 1997-04-01 Board Of Trustees Of The Leland Stanford Junior University Magnetic field inhomogeneity correction in MRI using estimated linear magnetic field map
US5627469A (en) * 1995-07-31 1997-05-06 Advanced Mammography Systems, Inc. Separation of fat and water magnetic resonance images
US6016057A (en) * 1998-04-17 2000-01-18 General Electric Company System and method for improved water and fat separation using a set of low resolution MR images
US6263228B1 (en) * 1998-08-27 2001-07-17 Toshiba America, Mri, Inc. Method and apparatus for providing separate water-dominant and fat-dominant images from single scan single point dixon MRI sequences
US6515476B1 (en) * 1999-06-24 2003-02-04 Ge Yokogawa Medical Systems, Limited Magnetic field inhomogeneity measurement method and apparatus, phase correction method and apparatus, and magnetic resonance imaging apparatus
US20030193332A1 (en) * 1999-12-24 2003-10-16 Shah Nadim Joni Magnetic resonance imaging method
US6707299B2 (en) * 1999-12-24 2004-03-16 Forschungszentrum Jülich GmbH Magnetic resonance imaging method
US20040010191A1 (en) * 2000-08-11 2004-01-15 Yumiko Yatsui Magnetic resonance imaging device and method
US7196518B2 (en) * 2000-08-11 2007-03-27 Hitachi Medical Corporation Magnetic resonance method which automatically forms water/fat separated images with different echo times and determines that proper phase unwrapping has been utilized
US20050122105A1 (en) * 2003-11-17 2005-06-09 Toshiba America Mri, Inc. Water-fat separation for fast spin echo imaging in an inhomogeneous field with progressive encoding
US7141972B2 (en) * 2003-11-17 2006-11-28 Toshiba America Mri, Inc. Water-fat separation for fast spin echo imaging in an inhomogeneous field with progressive encoding
US8260021B2 (en) 2007-07-20 2012-09-04 Siemens Aktiengesellschaft Method for correction of distortion in image data records recorded by means of a magnetic resonance scanner, as well as a computer program, image processing unit and magnetic resonance scanner for carrying out the method
US20090022385A1 (en) * 2007-07-20 2009-01-22 Dieter Ritter Method for correction of distortion in image data records recorded by means of a magnetic resonance scanner, as well as a computer program, image processing unit and magnetic resonance scanner for carrying out the method
US7812603B2 (en) * 2007-07-20 2010-10-12 Siemens Aktiengesellschaft Method for determining local deviations of a main magnetic field of a magnetic resonance device
US8199992B2 (en) 2007-07-20 2012-06-12 Siemens Aktiengesellschaft Method for correction of distortion in image data records recorded by means of a magnetic resonance scanner, as well as a computer program, image processing unit and magnetic resonance scanner for carrying out the method
US20090022384A1 (en) * 2007-07-20 2009-01-22 Dieter Ritter Method for correction of distortion in image data records recorded by means of a magnetic resonance scanner, as well as a computer program, image processing unit and magnetic resonance scanner for carrying out the method
US20090021258A1 (en) * 2007-07-20 2009-01-22 Siemens Aktiengesellschaft Method for determining local deviations of a main magnetic field of a magnetic resonance device
US20090256567A1 (en) * 2008-04-10 2009-10-15 Pelin Aksit Three-point method and system for fast and robust field mapping for epi geometric distortion correction
US8085041B2 (en) * 2008-04-10 2011-12-27 General Electric Company Three-point method and system for fast and robust field mapping for EPI geometric distortion correction
US9030201B2 (en) 2011-01-27 2015-05-12 Siemens Medical Solutions Usa, Inc. System and method for independent manipulation of a fat and a water component in magnetic resonance imaging
US9983284B2 (en) 2011-12-29 2018-05-29 Koninklijke Philips N.V. MRI with dixon-type water/fat separation and prior knowledge about inhomogeneity of the main magnetic field
US9256977B2 (en) 2012-02-01 2016-02-09 Siemens Medical Solutions Usa, Inc. System for reconstruction of virtual frequency selective inversion MR images
US20160131727A1 (en) * 2014-11-11 2016-05-12 Hyperfine Research, Inc. Pulse sequences for low field magnetic resonance
US10591561B2 (en) * 2014-11-11 2020-03-17 Hyperfine Research, Inc. Pulse sequences for low field magnetic resonance
US10955500B2 (en) 2014-11-11 2021-03-23 Hyperfine Research, Inc. Pulse sequences for low field magnetic resonance
US10866293B2 (en) 2018-07-31 2020-12-15 Hyperfine Research, Inc. Low-field diffusion weighted imaging
US11333726B2 (en) 2018-07-31 2022-05-17 Hypefine Operations, Inc. Low-field diffusion weighted imaging
US11510588B2 (en) 2019-11-27 2022-11-29 Hyperfine Operations, Inc. Techniques for noise suppression in an environment of a magnetic resonance imaging system

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ATE65847T1 (de) 1991-08-15
JPS6272346A (ja) 1987-04-02
EP0210038B1 (fr) 1991-07-31

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